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Rubber domes

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Overview

The vast majority of keyboards manufactured today use rubber domes. These are domes moulded out of silicone rubber that serve as the return spring for each key. Rubber domes are cheaper and in many cases quieter than metal spring switches. Modern rubber dome keyboards are almost universally membrane keyboards; the rubber dome also transfers the keystroke force from the operator’s finger onto the membrane assembly. Rubber dome keyboards may have a discrete rubber dome within each key, or they may have all the domes formed in a single moulded sheet. Scissor switch keyboards, used in most notebook computers and some desktop keyboards, have discrete domes, while more traditional-style desktop keyboards typically use a single rubber sheet.

In the strictest sense, a rubber dome is not a dome, as the top of a rubber dome is either a more complex shape or a hole. Keyboards where the top of the dome must press on a part (such as a membrane assembly or capacitive sense spring) or contain a sensing component will have an enclosed top, the inside of which may bear a conductive surface or conductive foil. Some other keyboard designs require that the plunger pass through the centre of the rubber dome, so there is a large hole in the top. Rubber domes where the top is absent are colloquially referred to as “buckling rubber sleeves”, where the term buckling is likely borrowed from IBM’s buckling spring design.

Additionally, although rubber domes are typically orientated upwards (as in a dome), they can also be used upside down (for example the “buckling rubber sleeves” in Mitsumi’s KPQ and KPR keyboards), and Jelco’s JKS-91 switch appears to use a dome mounted facing inwards.

Rubber domes collapse under pressure, and this collapse tends to be sudden rather than gradual. This sudden collapse provides the tactile feedback.

Rubber domes have the advantage of a rounded tactile peak, compared to the often sharp and jarring peak or step of many mechanical designs. This affords rubber dome keyboards a softer and more comfortable tactile feel than that of Cherry or Alps mechanical keyboards. As the plunger or keycap comes to a stop resting on the collapsed dome, the impact of reaching the end of travel position is cushioned compared to keyboards without a rubber actuator. This cushioning also cuts down one some of the typing noise, and the lack of metal springs removes the ringing sound (“ping”) associated with metal spring keyboards. While scissor switches are extremely quiet (almost to the point of being silent) rubber dome keyboards vary in noise level and still tend to have a level of clatter.

The chief disadvantage of rubber dome keyboards is that the behaviour of silicone rubber changes with age, which limits the lifetime of the keyboard to the amount of time until it becomes too difficult to type on. Typically, the rubber domes stiffen with age, requiring ever increasing effort to press keys; very old keyboards can require excessive force to operate. More unusually, the rubber can become limp, to the point of creating sensory deprivation from how much it absorbs the interaction force from your fingers. Rubber domes can also split from fatigue, which is a significant problem with vintage hardware, as it is far harder to replace a dome than it is to replace a coil spring. Another concern with rubber dome keyboards is that most of them have much less overtravel than other switch types.

The major alternative to rubber domes as an actuation method for membrane keyboards is spring over membrane, although other alternatives exist. Rubber domes can also be used in discrete switches, PCB-based conductive keyboards and in capacitive keyboards.

Terminology

Rubber dome
A single dome formed from silicone rubber, that acts as a return spring and actuator; this can be a discrete part, or formed as part of a continuous sheet.
Rubber sheet
A sheet of silicone rubber, with a dome-shaped moulding at each key position; this term is used by Cherry in their RS line of keyboards (e.g. RS 3000, also known as G83-3000)
Buckling rubber sleeve
This is a community term for a rubber dome that sits around the plunger; it is not a complete dome, but it operates the same. Buckling rubber sleeves can be found in an upside down orientation, making them closer to a dish than a dome. The term “sleeve” is something of a misnomer considering the conical or hemispherical (rather than cylindrical) form and the lack of contact between the “sleeve” and plunger, but the term has stuck.

Design

The exact feel of the keys depends in part on the profile of the domes. In Electronic Engineers Master 1985–86 Volume 2 (page B·1734), Moxness illustrated five different profiles of rubber key, along with the force curve that each one would produce:

Implementation

Rubber domes can be used in both discrete and non-discrete switch designs. The vast majority of rubber dome keyboards are non-discrete: the rubber domes or rubber sheet is paired with a single-piece moulded housing with guide shafts for the plungers or keycaps, or—with scissor switch keyboards—glued to the top membrane. In modern keyboards, rubber domes are typically used to apply operating pressure in membrane keyboards, but in some older keyboards, such as various BTC models, the inside of the dome bears a conductive material that bridges exposed areas on a PCB. Topre keyboards use a small, low-force conical spring as a variable capacitor, that is in turn operated by a rubber dome inside the key.

At least two manufacturers produced fully self-contained switches, that could be soldered to a PCB, with a conductive rubber dome inside. Alps Alpine (formerly Alps Electric) have a long history of so-called elastic contact switches formed in this manner, including at least five series of keyboard switches (KED, KEH, SKEW, SKPA and an unidentified DIN-compliant PCB-mounted series). SMK produced a keyboard switch of the same design, that was sold rebranded as Maxi-Switch.

History

Although rubber domes are most commonly associated with keyboards made in the 1990s onwards, the idea of using a conical rubber form as a return spring is not a new idea. US patent 3767022 “Return spring key stem boot” filed in April 1970 by Singer specifically refers to its “breakaway feel” and shows the force curve that results from its use. Singer’s design is what would be considered a “buckling rubber sleeve” in that it fits around the plunger, with a hole in the centre for the keystem. Such a keyboard has yet to be seen. The rubber dome itself was featured in an earlier patent, US patent 3465099 “Optical encoder”, filed in September 1967 by Friden, who had already been acquired by Singer in 1965. The Friden and Singer patents are both for the same general design of keyboard (specifically photoelectric encoder), but it seems that Singer deemed the design worthy of a patent of its own. The following diagram in the rubber boot patent shows the force curve:

Controls Research Corp filed US patent 3965399 “Pushbutton capacitive transducer” in March 1974 for a capacitive switch design, and this too features a conical rubber form inside the switch. The term “tactile feel” appears in the patent in a reference to unspecified products already known:

Such prior art devices have also attempted to provide a tactile feel when mechanical motion is applied to the transducers to cause a change from one state to another to enable an operator to feel “comfortable” in all substantial respects when operating the same …

There are unfortunately no further details about what implementations were already on the market at the time that the patent was filed. In relation to the use of the rubber dome in the product in question, the patent is clearer:

The shaft upon which the flexible film and key are disposed is biased toward deactuated position of noncontact with the capacitance forming member through the use of a frusto-conical biasing means comprised of elastomeric material which not only serves to maintain the shaft in a deactuated position at which it will “float” but also to provide a form of snap action in its biasing force from a position of biasing the shaft outwardly away from the capacitance forming member to a position which will allow full actuation and depression of the key …

The term “frusto-conical biasing means” here refers to the rubber dome inside the switch. The “snap action” description indicates that the tactile feedback provided by the rubber dome was intentional. Unlike with with Singer patent, the force curve is not included in the patent.

This design of switch is also yet to be observed.

Alps make extensive use of rubber domes within discrete pushbutton switches. The bottom of the dome is coated or impregnated with conductive material which bridges a pair of metal contacts when the switch is operated. The oldest type known to be designed for computer keyboards—KED Series—is known from as far back as 1980, being listed in the Boss DR-55 service manual (see non-keyboard part data). KED appears to have been replaced with KEH Series, which introduced a coil spring above a flatter dome, possibly to provide pretravel; unusually for a rubber dome switch, KEH Series is linear. KEH appears in the Roland TR-808 service manual from 1981. Japanese utility model S55-141429 from 1980 appears to cover KED series and is contemporary with the DR-55 service manual.

US patent 4354068 “Long travel elastomer keyboard”, filed by Texas Instruments in February 1980, depicts a full travel rubber dome-over-membrane keyboard with discrete domes. Whether Texas Instruments ever produced these keyboards remains to be discovered.

Another company who advertised discrete rubber dome switches was Maxi-Switch, who in December 1983 advertised their Series 6000 mechanical switches and 8000 Series internal-dome switches. Both types are SMK products. SMK advertised their JM-0400 series as far back as 1979. Confusingly, SMK listed JM-0400 series as containing “mechanical switches … reed type … pushlock switches, illuminated switches, and sealed membrane type units.” A “sealed membrane” type has yet to be observed, and there is presently no indication as to what SMK series Maxi-Switch used for their own 8000 Series.

Topre filed US patent 4584444 “Keyboard switch” for their capacitive switch system in September 1984. This design uses a conical metal coil spring to provide the variable capacitance, and a rubber dome as the return spring and as a source of tactile feedback. The Topre Corporation article on the Deskthority wiki depicts a prototype capacitive keyboard from 1981, with one photograph implying that it uses the exact same principle as a current Topre Realforce keyboard. An article cited by the wiki claims “キーボードへの取り組みはその中の1つで、1983年からスタートした。” (“the keyboard initiative is one of them, and started in 1983”); it is not clear from this when full-scale production began, when the product was first advertised, and when it went on sale. Topre capacitive keyboards use a PCB for the capacitive sensing.

Around the same time, Cherry in the United States advertised their Next Generation Keyboard, in particular in Computer Design magazine in March and June 1984. These keyboards were offered in linear and tactile variants; the linear types used a hybrid cylindrical–conical spring and the tactile types used a “dome-shaped silicone elastomer layer”. A cross-section drawing suggests that the rubber domes take the form of a single mat, which is what the aforementioned quotation in another advertisement implies. Cherry in Germany would at some point introduce the rubber sheet keyboards as their RS type (e.g. RS 3000, also known as G83-3000), but the origin of the RS/G83 line is not presently known. Also known as Full Travel Sealed Contact (not to be confused with the British and German Cherry FTSC designs), these were all membrane keyboards.

Another company to offer rubber dome keyboards in 1984 was Micro Switch, with their ST Series Silent-Tactile keyboards. In a July 1984 advertisement, Micro Switch claimed that “In increasingly noisy office environments, the quiet operation helps improve work atmosphere.” Micro Switch ST Series superseded SC Series capactive membrane and CT Series conductive membranes, offering both switching techniques in a single series. The only known change between SC/CT and ST was the exchange of metal leaf click actuators with quiet rubber dome actuators.

Advanced Input Devices (AID) advertised their EKI and EKT keyboards in 1984. These keyboards were based on an “elastomer one-piece switch mat”, which contained conductive domes placed over a PCB. AID would retain this as their approach to full-travel keyboards. The best-known AID keyboards are the two varieties of IBM PCjr keyboard.

Further details will emerge with ongoing research.